A new type of radar will help airliners to see and avoid the invisible threat.
When planes are flying at high altitudes far above thunderstorms, the air they cruise through may be full of tiny ice crystals. Normally, it’s pretty hard to tell. The ice crystals, no bigger than grains of flour, just blow through the engines harmlessly. Until they don’t, and by then you could already be in deadly trouble.
Often enough, the ice crystals in these areas of High Ice Water Content (HIWC) will bounce off cold surfaces like aircraft wings, but sometimes they partially melt and stick to warm warm ones like the inside of jet engines. How this happens, exactly, isn’t well understood but the affects are pretty straightforward. When a jet engine has a reduced air intake, it automatically reduces the amount of fuel it injects, a process called “rollback.” A pilot’s first notice will be a warning as the engine gradually dials itself back to idle and thrust drops to zero. That’s a bad scene for obvious reasons.
But rollback isn’t the only problem this ice can cause. Chunks of ice can break off inside the engine and cause all sorts of damage on the way out. Maybe most insidiously, crystal icing may block the openings to airspeed sensors, giving pilots bad data about how fast the plane is going. That was what happened to Air France Flight 447 when bad speed data (and a very poor response to it) led to a crash that killed all 228 people on board.
It’s no surprise that a major international effort is now under way to understand and detect crystal icing. The European High Altitude Ice Crystals (HAIC) project is coordinating with Boeing, Airbus, and NASA’s High Ice Water Content program to develop computer simulations of the icing process, and even build a giant laboratory to simulate icy conditions on full-size jet engines. But one of the most important aspects is developing instruments to detect crystal icing before it harms an aircraft. For the time being, instruments that detect icing conditions only work when an aircraft is already flying through them.
Right now, instruments that detect icing conditions only work when an aircraft is already in them.
Weather radar is good for detecting more normal icing conditions, which occur when an aircraft flies through freezing rain below 20,000 feet. The rain is easily visible on weather radar, so planes can avoid it. But crystal icing conditions are too subtle to spot the same way.
“Very fine ice water particles or ice crystals have very low radar reflectivity at the wavelength of our existing weather radars. This results in very small signal levels to be detected in the radar,” says Jan Lukas, senior scientist from Honeywell Aerospace’s Advance Technology function.
This does not mean that the ice crystals can’t be detected by radar, just that current radars are not tuned to pick them up. On most radars, the signal from tiny ice crystals is lost in the background noise, but by finding out what the ice crystal’s signature looks like the researchers believe they can highlight it when it appears.
On most radars, the signal from tiny ice crystals is lost in the background noise.
Researchers at Honeywell have been working on this as part of the HAIC effort. Their work has focused on the company’s RDR-4000 radar, which is already installed in many aircraft and provides a 3D weather map of the sky ahead.
A prototype of this upgraded radar was flown on Airbus A340-300 aircraft in Darwin in Australia and Saint Denis in Reunion in the Indian Ocean in January this year, areas picked for the frequency of violent storms that could lead to crystal icing. During the test flights, an aircraft equipped with special sensors to detect icing flew though areas of suspected HIWC while the radar aircraft scanned from a distance. This allowed researchers to confirm that ice crystals were present and match this with the radar returns.
“This is the first time such a demonstration has taken place and therefore establishes the state of the art,” says David Vacanti, senior engineering fellow at Honeywell Aerospace. “These flight tests have produced exceptionally encouraging results that show our RDR-4000 radar can provide the ability to detect HIWC at ranges of several tens of miles “
Practically, several tens of miles means minutes of advance warning, enough for a pilot to steer around the hazard just like a garden-variety thunderstorm. Better yet, the changes required to update the radar are minimal.
“We are working on software upgrades that can make software/firmware upgrades feasible in the very near future,” says Lukas.
It’s all looking good but there will still be some time before the tech makes it onto more planes, not for any technical challenge but due to a need for certification. Industry bodies will need to define the a whole series of metrics for ice crystal detection and then develop some sort of test to make sure that radars can meet those standards.
And those standards can’t come fast enough. Right now, there is roughly one crystal icing event per month, the sooner radar can be fielded the less chance there will be of another disaster.